1. Field of the Invention
The present invention relates to positive resist compositions. More specifically, it relates to a positive resist composition which is useful for forming fine patterns in the field of manufacturing electronic parts such as semiconductor devices and liquid-crystal display devices, and which can achieve a resist pattern exhibiting photosensitivity less variable relative to its thickness, superior shape-profiling ability, small thickness reduction even when the pattern is fine, especially when the pattern width is 0.35 .mu.m or less, ability to cope with irregularities in the exposure value since it has a wide focal-depth range, and reduced size-deviation from the mask size.
2. Description of the Related Art
Hitherto, in the manufacture of electronic parts such as semiconductor devices and liquid-crystal display devices, photolithography has been employed as a pattern-transferring process to form a pattern on a substrate. In the photolithographic process, a resist is applied onto a substrate such as a silicon wafer or a glass sheet using a spinner or the like and then dried, subsequently, the resultant is irradiated with an active light beam through a mask and then developed to transfer the mask pattern onto the substrate, and etching is performed. In the early manufacture of electronic parts as listed above, negative photoresists were used in combination with contact exposure to form etching masks for such pattern-transferring processes. Negative photoresists have, however, limited resolution due to swelling during development, and contact exposure damages the masks. Due to such drawbacks, the combination of a positive photoresist and reductive projection exposure, which exhibits superior definition, is employed instead of the aforementioned combination.
In general, a resist composition comprising an alkali-soluble resin and a quinonediazide-group-containing photosensitive compound dissolved in a solvent is used as the above-described positive photoresist. Such quinonediazide-group-containing compounds are, however, less soluble in conventional solvents. Due to this, the application of such a resist composition to a silicon wafer or a glass substrate is accompanied by coating irregularities, and therefore, a uniform coating film can rarely be obtained.
In view of such drawbacks, a large number of solvents for positive photoresists have been developed recently. The following are examples of such solvents:
Cyclopentanone (Japanese Patent Laid-Open No. 59-155838);
Monooxymonocarboxylates (Japanese Patent Laid-Open No. 62-123444);
Mixture solvents of cyclic ketones and alcohols (the specification of U.S. Pat. No. 4,526,856);
Propylene glycol monoalkyl ether acetates (Japanese Patent Laid-Open No. 61-7837);
Mixture solvents of organic solvents having vaporizing rates of 1 to 24 and organic solvents having vaporizing rates of 25 to 150, such as the mixture solvent of .gamma.-butyrolactone and 2-heptanone, wherein the vaporizing rates are measured according to a weighing method assuming the vaporizing rate of n-butyl acetate as 100 (Japanese Patent Laid-Open No. 4-362645);
Specific keto-ether solvents having boiling points of 130 to 170.degree. C., such as 4-methoxy-4-methyl-2-pentanone (Japanese Patent Laid-Open No. 2-247653);
Ethyl pyruvate (Japanese Patent Laid-Open No. 4-362646); and
Mixture solvents comprising specific combinations of ethyl lactate, n-amyl acetate and 2-heptanone (Japanese Patent Laid-Open No. 5-34918).
The above-listed solvents are, however, still unsatisfactory as positive resist compositions used in the manufacture of electronic parts which have been more finely processed, since a resist pattern formed using a positive resist solution prepared with such a solvent has photosensitivity readily variable relative to its thickness, exhibits marked thickness reduction when the pattern is fine, cannot cope with irregularities in the exposure value due to its narrow focal-depth range, and is rarely faithful to the mask size. Accordingly, there is a strong demand for the development of a more practical solvent for positive resists.